So, i was flipping through the strategy guide and I noticed that you can summon friendly NPCs with white stones, but it said nothing about online co-op, and I haven't seen any co=op summon stones my entire playthrough. (granted, i'm not that far.) Did they remove the co-op function?

You need to be in Human form before you can call for help. There's a knight after the Taurus Demon that gives you the item you need to offer up help in other players' games.

You have to be human to summon phantoms so without any humanity you will not be able to summon anything. However you don't get to choose who you summon. You will get a random stranger, that is part of the way this game works. Also I suspect there is an item needed to do it as well, so you might not have seen it yet.